Hyperosmolality regulates UT-A6 urea transporter expression in the Caco-2 cell line.

Gastrointestinal facilitative urea transporters play a significant role in the urea nitrogen salvaging process, which supports the symbiotic relationship between mammals and their gut microbial populations. UT-A6 urea transporters have been previously reported in the human gastrointestinal tract, specifically in the colon. As renal UT-A transporters can be regulated by external osmolality, this study investigated whether UT-A6 expression could also be regulated in this manner. Initial end-point RT-PCR experiments confirmed UT-A6 expression along the human gastrointestinal tract (colon > small intestine ≫ stomach) and also in the Caco-2 intestinal cell line. Using Caco-2 cells exposed for 24 hours to changed external osmotic conditions (from 350 to 250, 500, or 600 mOsm), end-point PCR suggested UT-A6 expression increased in hyperosmotic conditions. Using quantitative PCR, it was confirmed that 24 h exposure to 600 mOsm stimulated a significant ~15-fold increase in UT-A6 expression (P < 0.001, N = 5, ANOVA). Finally, inhibitory experiments suggested that protein kinase C and calcium were involved in this hyperosmotic-stimulated regulatory pathway. In conclusion, these data demonstrated UT-A6 expression was indeed regulated by external osmolality. The physiological significance of this regulatory process upon gastrointestinal urea transport has yet to be determined.